This invention relates to dot percentage measuring devices.
The shading of a continuous tone copy such as a photograph is expressed by using dots of different sizes in printing. In this method, the continuous tone copy is photographed through a screen or a contact screen to obtain a film called a screen negative or a screen positive in which the shading of the copy is expressed by dots different in size, and a printing plate is formed from this film.
A term "dot percentage" as used herein is a percentage of the total area of dots with respect to a unitary area in such a screen positive or a screen negative or a dot-printed matter. Accordingly, the value of dot percentage will greatly affect the tone and gradation of a printed matter.
When an obtained dot percentage is different from a predetermined one, it may be corrected by subjecting the screen positive or the screen negative to "reduction" which is one of the methods of correcting a dot percentage. In the reduction, the photographed film is washed with a so-called "reducer" to reduce the density of the picture. In the case of a dot-printed matter, a portion around a dot, which is low in density and is called "a fringe", on the film becomes transparent by the reduction process; that is, the dot percentage is decreased to correct the values of the printed matter. Thus, in printing with dots, the dot percentage is one of the important factors which determined the quality of a printed matter, and accordingly it is essential to control the dot percentage throughout the printing process from makeup to printing.
In an off-set retouch process, it is necessary to measure dot percentages after reduction has been carried out, and to ensure the configurations of dots, especially whether the dots are deformed or not.
In the case where the dot area becomes considerably smaller by excessive reduction or where measurement result indicates smaller dots, it is necessary that the dots are photographed again and are compared with the previous ones. Heretofore, dots are, in general, evaluated by visual inspection. However, the visual inspection is disadvantageous in that it involves personal errors and therefore it is necessary to provide a skilled person for the visual inspection. Since halftone-photographed dots are soft dots including fringes, the inspector will read the size of dots including fringes through his sense of sight, and accordingly the size of dots thus read is larger than the real size of dots.
Furthermore, although the data actually required in a reduction process is not the absolute value of dot percentage, but rather the difference between the dot percentage before reduction and that after reduction, no device for indicating this latter amount has as yet been provided.
A conventional dot percentage measuring device of this type is shown in FIG. 1(A). A light emitting section 3 is provided inside a measuring table 2 on which an object 1 to be measured, such as a film, is placed, and a light receiving section 4 is movably provided above the measuring table 2 in such a manner that it can confront the light emitting section 3. The light receiving section 4 is made up of a cylindrical head 4a which is open at one end, and a light receiving element 4b such as a photo-electric conversion element. In measuring a dot percentage with this conventional measuring device, the object 1 to be measured is placed on the measuring table 2, and a portion to be measured of the object 1 is positioned at the measuring section of the device (or on the line connecting the light emitting section 3 and the light receiving section 4). Then, the light receiving section 4 is lowered in the direction of the heavy arrow P to confront the light emitting section 3 as shown in FIG. 1B, so that the upper opening edge of the head 4a is in close contact with the portion to be measured of the object 1. Under the conditions that the external light is shielded, the light emitting section 3 emits a predetermined intensity of light to irradiate the lower surface of the object 1 in the direction of the shorter arrow Q shown in FIG. 1(B) so that the light receiving element 4b in the light receiving section 4 receives light passed through the object 1, and the dot percentage of density of the object 1 is displayed by a display section (not shown) connected electrically to the light receiving element 4b.
However, the conventional measuring device is disadvantageous in the following points: The construction of the measuring device is intricate because the device is so designed that the light receiving section is movable towards the light emitting section as was described above. The operation of the measuring device is intricate and accordingly troublesome because positioning the measurement portion of the object 1 must be carried out before measurement and the upper opening edge of the head 4a must be brought into close contact with the measurement portion of the object 1. Therefore, in the case where a dot percentage of density measured should be corrected by reduction, it is necessary to carry out the following steps: After the object 1 is removed from the measuring table 2 and is subjected to reduction process by using a washing table or the like, the object 1 must be placed on the measuring table 2 again. This will increase labor and decrease work efficiency.
Measurement of a dot percentage is carried out in the processes of halftone photography and development also. The dot percentage measurement can be carried out with the aforementioned conventional measuring device; however, the conventional measuring device is not suitable for frequently performing the measurement for the following reasons: Since the light measuring section of the conventional device is provided at the lower portion of the case thereof, the light measuring section is out of the operator's field of vision, which makes it difficult to place the light measuring section at a portion to be measured of an object. A film density may be measured by performing the positioning only once; however, in measuring a dot percentage as described above, the operator has to assume an uncomfortable position to direct the light measuring section to the portion to be measured. This is undoubtedly troublesome for the operator.
Two practical methods of measuring a dot percentage have been known in the art. In the first method, the shading of a dot film is converted into an electrical signal by means of, for instance, a vidicon to measure the area of a portion of the film where the density is higher than a predetermined value, thereby to measure the dot percentage. In the second method, the light transmittance of a portion to be measured of the dot film is measured to obtain the dot percentage. The first method is advantageous in that it is not affected by fringes around dots and therefore its measurement can be achieved theoretically with high accuracy; however, it is disadvantageous in that the measuring device is bulky and expensive, and therefore not practical. The second method is also disadvantageous in that it is greatly affected by the fringes, and therefore errors are liable to be involved in measuring a halftone-photographed film having a large fringe area.
In general, the error due to the fringe is small or zero with dot percentages of about 0% and about 100% and great with a dot percentage of about 50%. In order to correct this error, in the second method, the multiplying factor of the display section is finely adjusted, or transmission light to be measured is subjected to photoelectric conversion, and then the amplification factor of the resultant electrical signal is finely adjusted. In this correction method, a film having a known dot percentage, preferably about 5%, is prepared, and the fine adjustment is carried out so that the measurement value of the film coincides with the known value. With the film subjected to this correction, the errors due to the fringe are relatively satisfactorily corrected with dot percentages of about 0% and about 100%, but they are still large with a dot percentage of about 50%, as indicated in FIG. 2. The relation of the dot percentage to the light transmittance is not linear because of the influence of fringes, that is, it is expressed by a characteristic curve which is curved in the vicinity of 50% as shown in FIGS. 3 and 4. Nevertheless, the conventional correction method assumes that the relation of the dot percentage to the light transmittance is linear, and performs fine adjustment of this straight line slope. Accordingly, in the conventional correction method, a relatively great error remains in the vicinity of 50% when referred to the errors at dot percentages 0% and 100%.
When a dot film is subjected to reduction by using an iron chelate group reducer, its portion subjected to the reduction is colored yellow-brown. When this film is measured in the second method the light transmittance is affected by the colored portion, and the measured dot percentage involves errors. These errors depend on the amount of reduction, and therefore the correction cannot be achieved by the measuring device according to the second method.